All disk files require some means of determining the radial position of the read-write heads over the disks so that the heads can be accurately positioned over any desired track. Typically this is done by putting servo information on one or more of the disk surfaces for reading by magnetic or optical read heads. Some disk files have servo information only on a dedicated surface of one disk in a disk stack. However, more recently the trend is to store this servo information interspersed with the data stored on each disk surface. This latter approach is preferred because it can be implemented at low cost without extra components beyond those required for storing data and because it provides the servo information at the data surface being accessed, thereby eliminating all thermal sources of track misregistration (TMR).
In a disk file in FBA format, each circumferential track on the disk is divided into a number of sectors. Each sector consists of a factory written portion and a user written portion. The factory written portion is the servo region which contains the servo information and timing information required to mark the start of the sector. The user written portion consists of a data region containing the actual data to be stored and possibly an identification (ID) region which identifies the sector and marks bad sectors. A large amount of overhead, in which no user data can be stored, is associated with each sector.
Fast servo systems require frequent position measurements; i.e., a high servo sample rate. In sectored servo arrangements, this is achieved by dividing each track on the disk into a large number of small sectors. As a result, a large fraction of the potential information capacity of the disk file undesirably is used for the overhead associated with each sector. For example, if the servo sample rate is doubled, it will double all of the non-data overhead.
The most pertinent prior art of which applicants are aware is an article in the Spring 1988 issue of Computer Technology Review at pp. 45-48. This article describes a method for reducing this overhead to a very limited degree in an arrangement which essentially converts a FBA disk into a CKD format. To accommodate variable length data records extending over a number of data sectors, an equivalent to the ID region is put in only the first data sector of the record. This saves some overhead if the data records are long, but none, of course, if the record is less than a sector in length. Each sector may be independently written after leaving the factory; thus, complete resynchronization is required at each data sector. All other overhead fields including full automatic gain control (AGC), write-to-read and read-to-write recovery and synchronization fields, are therefore still required.
There is a need for an improved method and means, transparent to the user, for increasing the servo sample rate in either a disk file or a tape drive using the FBA format, and for achieving this increase with a minimal increase in the overhead.